1. Field of the Invention
This invention relates to an apparatus for manufacturing heat transfer tubes for use in evaporators and coolers of freezers, air conditioning units, refrigerators, and the like.
2. Description of the Prior Art
Among the prior inventions, one closely concerned with this invention is disclosed in the specification of Japanese Patent Publication No. 31863/74.
The specification describes a method of forming numerous rhombic or rhomboidal protuberances in a regularly spaced pattern on the inner surface of a pipe by first inserting a plug having a number of helical edges on the outer surface into a pipe with a smooth inner surface to form helical ribs and grooves thereon, and then inserting another plug having a number of helical grooves of a lead angle different from that of the first plug into the same pipe to cut and partially remove the previously formed helical ribs at regular intervals.
According to the method, the first step of operation for forming the helical grooves and the second step for partially removing the resulting helical ribs are separate and independent of each other. In the second stage the relative positions of the plug and pipe tend to be instable, resulting in dimensionally ununiform removal of the ribs. For this reason, the technique taught by the Japanese Pat. Publication No. 31863/74 cannot be resorted to where the relative positions of the plugs in the first and second steps are to be consistent.
The heat transfer surface structure to be provided in accordance with the present invention for the interior of a heat transfer tube is generally the same as the structure revealed in our copending U.S. Pat. application Ser. No. 586,930.